Sectored servo disk formatting

ABSTRACT

A sector on a disk contains position information used to determine position error of a transducer over the disk. The sector has a normal field with a first side and a second side. The first side is oriented opposite the second side on the disk. A first quadrature field is located on the first side of the normal field and a second quadrature field is located on the second side of the normal field.

This is a continuation of application Ser. No. 07/415,054 filed on Sep.29, 1989, abandoned as of the date of this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to positioning a transducer over a disk ina disk file data storage system. More particularly, the presentinvention relates to demodulation of position information signalsprovided by the transducer in response to position information read fromthe disk.

2. Description of the Prior Art

Disk file data storage systems contain one or more magnetic disks onwhich data is stored in concentric tracks. A transducer writes, ormagnetically encodes, the data on the track. The transducer is alsocapable of reading the magnetically encoded data from the tracks.

An electromechanical actuator operates within a negative feedback,closed-loop servo system. The actuator moves the transducer radially fortrack seek operations and holds the transducer directly over a track fortrack following operations.

A servo transducer reads position information from the disk and providesa position signal which is decoded by a position demodulator andpresented in digital form to a servo control microprocessor. The servocontrol microprocessor essentially compares actual radial position ofthe transducer over the disk with desired position and commands theactuator to move in order to minimize position error.

In one type of servo system, one disk surface is dedicated to containservo tracks which are encoded with servo position information. Theservo position information in the servo tracks is condensed to evenlyspaced sectors. A servo transducer flies over the servo sectors as thedisk rotates and produces a sampling effect. The actual positiontransducer information is updated at the end of each servo sectorthrough the use of track identification information and position errorinformation. The track identification number is prewritten into eachservo sector and serves as coarse transducer position information. Theposition error information is written in the servo sector and representsthe distance that the servo transducer is located from the center of thetrack. This position error information serves as fine transducerposition information.

The position error information is generally written in two fields. Oneis referred to as a quadrature field and the second is called a normalfield. Position error information obtained from the normal field or thequadrature field is called a normal or quadrature position sample. Bydecoding the position samples obtained from these two fields, theoff-track position of the transducer is determined relative to thecenter of the track. The position samples are typically decoded byintegrating the analog position signal provided by the transducer whichrepresents the position error information magnetically encoded on thedisk. The integrated signal is then converted to a digital signalrepresenting transducer position error.

In the past, the normal field and quadrature field, as encoded on themagnetic disk, were located adjacent one another. As the transducer flewover the normal field and then the quadrature field, the positionsamples from the normal and quadrature fields were decoded. However,radial movement of the transducer over the magnetic disk resulted in aslanted trajectory of the transducer with respect to the quadrature andnormal fields on the magnetic disk. The slanted trajectory causednonlinearities in the form of an apparent phase shift in the positionsamples decoded from the quadrature and normal fields. This phase shiftresulted in nonlinearities in the integrated position signal. Thenonlinearities also caused inefficient actuator control.

SUMMARY OF THE INVENTION

The present invention reduces nonlinearities in position signals used todetermine transducer position error. A sector on a disk containingposition information is used to determine position error of a transducerof the disk. The disk is encoded with a normal field that has a firstside and a second side. The first side is oriented opposite the secondside on the disk. A first quadrature field is located on the first sideof the normal field and a second quadrature field is located on thesecond side of the normal field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one of several possible servo systems ofthe present invention.

FIG. 2 is a view of a disk surface used in a sectored servo system.

FIG. 3A is an enlarged view of a portion of a servo sector from the diskshown in FIG. 2.

FIG. 3B is a diagram of the format of servo information in one servosector.

FIG. 4A shows the transducer output signal amplitude as a function oftrack radius for one servo sector.

FIG. 4B is a drawing showing the spatial relationship between thetransducer and magnetized areas on the disk.

FIG. 5 is a diagram used to mathematically describe the amplitude of thetransducer output signal as a linear function of the distance of thetransducer center from the track center

FIG. 6 is a detailed block diagram of the servo position demodulator.

FIG. 7 is a timing diagram showing PES capacitor voltage during oneservo sector.

FIG. 8 is a timing diagram showing the relationship between events inthe servo position demodulator.

FIG. 9A is a graph showing the output of a multiplier in the servoposition demodulator as a function of track radius.

FIG. 9B is a diagram showing magnetized bits in a servo sector and theposition of the transducer which corresponds to the positions graphed inFIG. 9A.

FIG. 10 is a diagram which illustrates the relationship between thefinal voltage on the PES capacitor and the off-track error of thetransducer.

FIG. 11 is a graph of the analog voltage PES1 versus track radius of thetransducer in one servo sector.

FIG. 12A is a more detailed block diagram of the DC null loop.

FIG. 12B is a timing diagram showing the relationship between keysignals in the DC null loop.

FIG. 13A shows the concept of a total position word.

FIGS. 13B1, 13B2 and 13B3 are diagrams showing the PES voltages and theleast significant bit of the raw track identification number as afunction of transducer position.

FIG. 13C is a truth table giving the correction number for each caseshown in FIG. 13B.

FIG. 13D is a block diagram of the track identification correctionfeature.

FIG. 13E is one preferred implementation of the track identificationcorrection feature.

FIGS. 14A1, 14A2 and 14A3 show the track crossing plots for the PESfields in a servo-on-data system.

FIG. 14B shows the relationship between the transducer position in theservo sector and the track crossing plots.

FIG. 14C shows the position of the transducer over one track in theservo sector.

FIG. 15A shows transducer trajectory over a servo sector caused byradial movement of the transducer as the disk rotates.

FIG. 15B is a plot showing the effect of radial velocity of thetransducer on the PES track crossing plots.

FIG. 15C shows the corrected PES track crossing plots after beingcompensated for radial velocity of the transducer.

FIGS. 16A1, 16A2 and 16A3 show the track crossing plots for the PESfields in a dedicated servo system.

FIG. 16B shows the relationship between the transducer position in theservo sector and the track crossing plots.

FIG. 16C shows the position of the transducer over one track in theservo sector.

FIG. 17 shows a servo system having multiple independent actuators.

FIG. 18 shows a top view of a disk in the servo system shown in FIG. 17.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 1. Introduction

FIG. 1 is a block diagram of negative feedback transducer positioningsystem 8. A stack of magnetically encodable disks is shown generally at10. An electromechanical actuator (or E-block) 12 is used to position aservo transducer 14 and several data transducers 18 radially withrespect to a servo disk 16 and data disks 17, respectively. The servotransducer is positioned over a desired track on servo disk 16 where itis held while data transducers 18, which are also connected to E-block1, perform read or write operations.

Servo transducer 14 reads position information from servo disk 16. Servotransducer 14 provides position signals to a preamplifier 20 where it isamplified and provided to servo position demodulator 22. Servo positiondemodulator 22 decodes the position information and presents it, indigital form, to servo control processor 24. Servo control processor 24compares the decoded position signal received from servo positiondemodulator 22 with a desired position signal to determine a transducerposition error. The transducer position error represents the differencebetween the actual position of transducer 14, indicated by the decodedposition signal, and the desired position indicated by the desiredposition signal.

Servo control processor 24 then generates a position correction signalwhich is converted to an analog signal in digital-to-analog (D/A)converter 26 and applied to actuator 12 through power amplifier 28. Theposition correction signal causes actuator 12 to move the transducers 14and 18 radially with respect to the disks 16 and 17 in order to minimizethe transducer position error.

2. Disk Formatting

In the preferred embodiment shown in FIG. 1, one disk surface of servodisk 16 is dedicated to contain servo tracks. This surface is shown inFIG. 2. The servo position information in the servo tracks is notcontinuous but is condensed into evenly spaced sectors. This is known asa sectored servo system. Servo transducer 14, flying over the servosectors as disk 16 rotates, produces a sampling effect in which theactual, decoded position information provided to servo control processor24 is updated at the end of each servo sector.

FIG. 3A shows an enlarged portion of a servo sector on servo disk 16.FIG. 3B shows the format of servo information, represented by black andwhite bars, in one servo sector on disk 16. The black and white barsrepresent oppositely magnetized areas on a surface coating of servo disk16. The oppositely magnetized areas are recorded close enough togetherso that the position signal generated by servo transducer 14 isgenerally sinusoidal.

FIG. 3B shows that one servo sector contains several distinct fields.The fields include a phase locked oscillator/automatic gain control(PLO/AGC) synch field, a track identification (track ID) field, threePosition Error Sample (PES) fields and an index field. FIG. 3B alsoshows the magnetized areas for three tracks A, B and C on servo disk 16where the outer radius and inner radius of the tracks is indicated. Thedirection of the transducer velocity and the track center of each trackis indicated by dashed arrows A, B, and C, respectively.

As transducer 14 flies over track A, B, or C on disk 16, it generates agenerally sinusoidal position signal. The sinusoidal position signalrepresents the magnetically encoded information shown in FIG. 3B.

FIG. 4A illustrates the position signal generated by transducer 14 as afunction of five radial transducer positions along a single track (forexample track B) on disk 16 where position 3 represents the center ofthe track. Positions 1 and 5 represent off-center positions where thecenter of transducer 14 is radially aligned with, and flying over theouter radius or inner radius of track B, respectively. If transducer 14were tracking along position 1, a position error equal to the radialdistance between position 1 and position 3 would exist. Similarly, iftransducer 14 were tracking along position 5, a position error equal tothe radial distance between position 3 and position 5 would exist.

Positions 2 and 4 also represent off-center positions. However, theoff-center position error is not as large as the error associated withpositions 1 and 5. FIG. 4B shows a spatial relationship betweentransducer 14, the magnetized areas on disk 16, and each of thetransducer positions 1 through 5.

The information encoded in the PLO/AGC synch field is identical in phaseand frequency from track to track along disk 16. Therefore, FIG. 4Ashows that, as transducer 14 flies over this region, its output positionsignal is a constant frequency, independent of the particular track overwhich it is flying and independent of radial position on the track. Theinformation in this field is encoded in this way so that a phase lockedoscillator in servo position demodulator 22 can synchronize with theoutput of transducer 14.

In the track ID field, the track ID number is encoded as a Gray codewhere only one bit changes from track to track. Each track ID bit isrepresented by one dibit cell. The amplitude of a dibit depends on theradial position of servo transducer 14. If the dibit amplitude exceeds agiven threshold, in this preferred embodiment one half the maximumpossible amplitude, the dibit is decoded as a logic 1 in servo positiondemodulator 22. Otherwise, it is decoded as a logic 0. Therefore, n bitcells define 2^(n) servo tracks on the disk.

In the PES fields (PES1, PES2A and PES2B) the magnetic informationencoded on adjacent tracks is recorded 180° out of phase. This producesa checkerboard pattern of magnetization. The magnetic information in thePES fields is also recorded plus or minus 90° out of phase with respectto the magnetic information encoded in the PLO/AGC synch field.

The PES1 field is also called the normal field. The boundary between themagnetization tracks in the normal field are arranged so that they lieon the center line of data tracks. Therefore, when transducer 14 is overthe normal field, as transducer 14 moves radially further from centerposition 3 of track B, the amplitude of the position signal fromtransducer 14 is larger. Similarly, as transducer 14 moves radiallycloser to center position 3, the amplitude of the position signal issmaller.

The PES2 field (also known as the quadrature field) is split in half.Half of the quadrature field (the PES2A field) lies on one side of thenormal field and half (the PES2B field) lies on an opposite side of thenormal field. The quadrature field is encoded with a magnetizationpattern which is identical to that of the normal field but which isradially off-set from it by half a track width. Therefore, whentransducer 14 is over the quadrature field, the position signal providedby transducer 14 is at a maximum amplitude when transducer 14 is flyingover track center position 3 and at a minimum amplitude when it isflying over the off-center positions 1 and 5.

The information read from the normal and quadrature fields is referredto as a position error sample. The normal and quadrature fields eachproduce a separate position error sample. The amplitude of the positionsignal provided by transducer 14 representing the position error samplesis a linear function of the distance of the center of transducer 14 fromthe track center (position 3). This is shown theoretically in FIG. 5.

Transducer 14 is moving relative to disk 16 in the direction indicatedby arrow 30. Transducer 14 may be regarded as being composed of a set ofread elements of differential width operating in parallel, whose outputsignals are added together or integrated to produce the total outputvoltage of the position signal from transducer 14. For a first orderapproximation, it is assumed that the position signal from transducer 14is sinusoidal. Any fringing field effect is neglected. Since themagnetic pattern in the servo sector was originally written 180° out ofphase for adjacent tracks, the signal produced by a differential readelement, dy, will be defined as either plus or minus Vm Cos (wt). Thetotal output voltage from transducer 14 depends on how many differentialread elements produce +Vm Cos (wt) and how many produce -Vm Cos (wt).

To express this mathematically, let d equal the distance from the centerof transducer 14 to the nearest data track center (a boundary betweenthe +Vm Cos (wt) and -Vm Cos (wt) regions in the PES1 field). Distance dmay be positive (towards the inner radius) or negative (towards theouter radius). If b equals the width of transducer 14, then each readelement dy contributes a fraction equal to dy/b towards the total outputvoltage provided by transducer 14. The integral for the PES1 field asshown in FIG. 5 is solved as follows: ##EQU1## where b=core gap width oftransducer 14

Vm=maximum amplitude

d=off track error

w=frequency of position signal

The results show that the position signal of transducer 14 V(t) has anamplitude which is a linear function of the off-track error, d. Thiseffect can be seen visually in FIG. 4A where the signal amplitude in thePES1 field is proportional to the distance of the center of transducer14 from the track center, radial position 3. Also, the polarity of theposition signal generated by transducer 14 which corresponds to the PES1field indicates the direction of the off-track error, d.

In reality, the amplitude of the position signal of transducer 14 doesnot change linearly across the full track width. It becomes nonlinear atsome point near the outside 25 percent of the track, depending on thephysical dimensions of transducer 14 and its relationship to thedimensions of the written magnetized pattern.

However, the presence of the quadrature field (PES2) allows servoposition demodulator 22 to overcome this nonlinearity. FIG. 4A shows thequadrature relationship between the PES2A and 2B fields and the PES1field. The amplitude of the position signal provided by transducer 14 inthe PES2 field is at a maximum, and therefore in a nonlinear region whenthe amplitude in the PES1 field is at a minimum (and therefore in alinear region), and vice versa. Hence, either the quadrature or thenormal field is always in a linear region and there is always a linearrelationship between the position signal corresponding to one of the PESfields (PES1 or PES2) and the distance between the center of transducer14 and the track center (the off-track error) no matter where transducer14 is positioned across the radial width of a track on disk 16.

3. Demodulator Hardware Description

The demodulator hardware itself is described in this section. A detaileddescription of its operation is provided in section 4. FIG. 6 shows amore detailed block diagram of servo position demodulator 22 whichdecodes position information represented by the analog position signalfrom transducer 14. The position signal from transducer 14 is applied tofilter 32 where it is filtered to remove high frequency noise. Thefiltered signal is then applied to automatic gain control (AGC) loop 34.

a. Automatic Gain Control Loop

AGC loop 34 is comprised of voltage controlled amplifier 36, analogmultiplier 38, voltage to current converter 40 and current summingjunction 42. Analog switch 44 and capacitor 46 allow AGC loop 34 to beconfigured in an Acquire mode or a Hold mode (explained later).

An overall reference for the automatic gain controlled amplitude isprovided through filter 48 by a microprocessor (not shown). The overallreference determines the final position gain in volts per microinch.

The AGC loop 34 is initially calibrated by the microprocessor andcompensates for changes in input signal amplitude due to changes intrack radius, flying height and other media properties.

b. PEG and Track ID Decoding

Once properly amplified, the automatic gain controlled (AGC'd) analogsignal is transmitted to voltage comparator 50 which generates serialdigital data corresponding to the AGC'd analog signal. Comparator 50provides the serial digital data to synch detector circuit 52 and shiftregister 54.

Synch detector circuit 52 identifies the start of a servo sector andenables programmable event generator (PEG) 56. Programmable eventgenerator 56 produces timing and window logic signals required forposition demodulation in servo position demodulator 22.

Next, comparator 50 decodes track ID dibits as either logic 1 or logic 0depending on their amplitude. In this preferred embodiment, comparator50 has a threshold of one half the maximum automatic gain controlledamplitude. The serial digital track ID bits are shifted intoserial-to-parallel shift register 54 to produce a parallel track IDword. This track ID word is in Gray code and is converted to a binarytrack ID word by combinational logic 58. The binary track ID word islatched in latch 60 and provided to track ID correction logic 62 whichwill be explained in more detail later in the specification. The outputof correction logic 62 is a track identification signal or absoluteposition signal which identifies the particular track over whichtransducer 14 is flying.

Shift register 54 is also used to provide index information to indexpattern detector 64 which is not important to the present invention andwill not be discussed further.

c. Phase Locked Oscillator Loop

The AGC'd analog signal is also transmitted to phase locked oscillatorloop (PLO loop) 66. PLO loop 66 comprises analog switch 68, phasecomparator/multiplier 70, voltage-to-current converter (charge pump) 72,which includes balance circuit 74 for balancing the voltage-to-currentconverter 72 once per servo sector so that there is zero current outputfor zero voltage input, analog switch 76, filter 78, voltage controlledoscillator 80 and clock generator 82. PLO loop 66 locks onto the phaseand frequency of the analog signal provided by AGC loop 34. Then, analogswitch 76 is opened allowing PLO loop 66 to open. The signal provided tovoltage controlled oscillator 80 is then held constant. Hence, aconstant frequency results at the output of voltage controlledoscillator 80. Therefore, the frequency of feedback 120 from clockgenerator 82 is also constant.

d. PES Generator

The output of voltage to current converter 72 is also provided to PEScapacitor 84 through analog switch 86. Voltage is integrated oncapacitor 84 during synchronous demodulation of PES fields 1, 2A and 2B.This voltage is applied through buffer 88 to analog-to-digital (A/D)converter 90. The output of A/D converter 90 is provided to compositePES generator logic 92. Composite PES generator logic 92 compriseslatches 94 and 96, digital adder 98, multiplexers 100 and 102,multiplexer control logic 104, switchable invertor 106 and invertorcontrol logic 108. The output of composite PES generator logic 92 is acomposite PES word or fine position off-track error which represents thedistance that transducer 14 is tracking from the track center.

The composite PES word, together with the corrected track ID word, formone digital position word which is typically 19 bits long. The digitalposition word defines the radial position of transducer 14 on disk 16 ina continuous fashion down to approximately one or two microinches ofquantization.

e. DC Null Loop

PES capacitor 84 is also selectively connected to DC null loop 110comprised of A/D convertor 90, one bit latch 112, integrator 114, buffer116, analog switch 118 and buffer 88. DC null loop 110 drives theinitial voltage on PES capacitor 84 to a midpoint between upper andlower reference voltages applied to an internal resistor ladder in A/Dconvertor 90.

5. Operation a. PLO/AGC and Track ID Decoding

Operation of the positioning system is described with reference to FIGS.7 and 8. FIG. 7 is a timing diagram which shows the relationship betweenlogic control signals generated by programmable event generator 56, theanalog position signal and the PES capacitor voltage as transducer 14moves over one servo sector. FIG. 8 is a timing diagram showing therelationship between various signals related to the synchronousdemodulation of the fine position information in servo positiondemodulator 22 as transducer 14 moves along one servo sector.

As transducer 14 passes over the AGC/PLO synch field in the servosector, programmable event generator 56 closes switch 44 to close AGCloop 34. AGC loop 34 acquires the proper gain so that the amplitude ofthe analog position signal is compensated for amplitude changes causedby changes in track radius, transducer flying height, and other mediaproperties. Also, as transducer 14 flies over the AGC/PLO synch field,programmable event generator 56 closes analog switches 68 and 76 toclose PLO loop 66. When PLO loop 66 is closed, voltage controloscillator 80 locks onto the phase and frequency of the analog positionsignal provided by AGC loop 34. PLO loop 66 uses digital feedback signal120 which locks in at a 90° phase shift from the analog position signalprovided by AGC loop 34.

After transducer 14 has passed over the PLO/AGC synch field, analogswitches 44 and 76 are opened and both AGC loop 34 and PLO loop 66 entera Hold mode. While in the hold mode the amplification in AGC loop 34 andthe phase and frequency of VCO feedback 120 in PLO loop 66 remainconstant.

Next, transducer 14 passes over the track ID field in the servo sector.The serial digital track ID bits are formed by comparator 50 and shiftedinto serial-to-parallel shift register 54. The Gray code track ID bitsare converted into a binary code by convertor 58 and latched into latch60.

b. PES Decoding

Finally, transducer 14 enters the PES fields in the servo sector. Switch68 is closed applying the analog position signal to multiplier 70 whileswitch 76 remains open. The digital VCO feedback signal 120 ismultiplied by the analog position signal in multiplier 70. Since theinformation in the PLO/AGC synch field is recorded 90° out of phase withthe information in the PES fields, and since digital VCO feedback signal120 is held with a 90° phase shift, the multiplication performed atmultiplier 70 produces a full wave rectification of the analog positionsignal. This is shown by plot C in FIG. 8.

FIG. 9A illustrates the output of multiplier 70 for the five radialpositions of transducer 14 which were described earlier with referenceto FIG. 4A. Also, FIG. 9B shows the spacial relationship between thecenter of transducer 14 and the center of track B in radial transducerpositions 1-5. In the PES fields, the amplitude of output signalmultiplier 70 is proportional to the distance between the center oftransducer 14 and the center of track B. The polarity of the DCcomponent of the signal indicates the direction of the off-set. Hence,when the current provided by voltage-to-current converter 72 is appliedto PES capacitor 84, the current is integrated, producing a positive ornegative voltage ramp depending on the polarity of the current. Severalsuch voltage ramps are indicated at points 122, 124 and 126 in FIG. 8.FIG. 10 illustrates the general case where n cycles of current areintegrated to produce the analog voltage on PES capacitor 84corresponding to one PES field. The integral is solved as follows:##EQU2## where, n=# cycles measured

b=width of transducer 14

c=capacitance of PES capacitor 84

Vm=maximum AGC'd amplitude

gm=gain through charge pump 72

w=frequency of position signal.

The result shows that the final voltage on PES capacitor 84 is directlyproportional to the off-track error, d, since n, c, b, Vm, gm and w areall constants. The relationship between the final analog voltage on PEScapacitor 84 corresponding to the PES1 field and the off-track error, d,is graphically illustrated in FIG. 11. The voltage ramps for five radialpositions in one servo sector are shown as they relate to off-trackerror, d.

The final voltage on PES capacitor 84 is also proportional to thenumber, n, of cycles integrated. In the PES1 field, n cycles areintegrated and A/D converter 90 converts the analog voltage on PEScapacitor 84 into a digital signal which is latched in latch 94. For thePES2 fields, first n/2 cycles (for PES2A) are integrated and A/Dconverter 90 converts the value to a digital signal which is latched inlatch 96. Then, after the PES1 field is integrated, another n/2 cycles(for field PES2B) are integrated and another A/D conversion is made byA/D converter 90. The resulting digital signal is provided to adder 98where the digital signal representing the PES2A field is added to it toproduce the full PES2 position sample. The number of cycles, n, chosenshould be high enough to achieve the averaging of possible media defectsover several cycles. However, it must not be so high as to produce anoverly long servo sector since the length of the servo sector limits themaximum radial velocity of transducer 14.

The final voltage on the PES capacitor 84 is also proportional to thecapacitance of PES capacitor 84. Note that the same PES capacitor 84 isused to produce the analog PES voltage for both the PES1 and PES2position samples. After the A/D conversion of the final ramp voltage ofa PES measurement, the digital result is latched so the analog voltageon the PES capacitor 84 can be reset, an the same PES capacitor 84 canbe re-used for the next PES measurement. This eliminates the possibilityof position error due to capacitance mismatch which would result if twoor more capacitors were used to produce the analog PES voltage. Usingonly one capacitor to integrate the position signal for both thequadrature and normal fields increases the accuracy with whichtransducer 14 follows the track.

Accordingly, only a single A/D converter 90 is required to convert theanalog PES voltages to digital signals. This lowers the overall cost ofservo position demodulator 22. Also, the A/D conversion made on A/Dconverter 90 is made early in the processing chain in servo positiondemodulator 22. The addition of the PES2A and PES2B samples and theformation of the composite PES signal all take place in the digitaldomain. This minimizes the amount of analog circuits and theirassociated errors such as off-set errors, gain errors, linearity errorsand variation with temperature. Additionally, all of the digitalcircuits can be fabricated into one compact integrated circuit to savespace.

c. DC Null Loop

DC voltage off-sets in buffer 88 and analog switch 118 would create anoff-set error in the final digital composite PES signal. Therefore, DCnull loop 110 is provided. FIG. 12A shows a more detailed block diagramof null loop 110, and FIG. 12B is a timing diagram showing therelationship between signals occurring in DC null loop 110 As transducer14 passes over each servo sector, and before it reaches the PES fields,programmable event generator 56 closes analog switch L3 connecting PEScapacitor 84 to the output voltage of integrator 114.

Next, programmable event generator 56 generates a signal on output L7causing A/D converter 90 to perform a first A/D conversion. A/Dconverter 90 comprises a resistor ladder connected between referencevoltages VREF+ and VREF-, 15 comparators including most significant bitcomparator 130, and logic decode 132 which decodes a most significantbit of A/D converter 90. Resistors R1 and R2 which represent theresistor ladder condensed into two resistors, act as a voltage dividercreating a voltage midpoint Vmidref at node 129.

After the first A/D conversion, the most significant bit of A/Dconverter 90 is decoded and latched into single bit latch 112. The valueof the latched most significant bit depends on whether signal Vin isgreater or less than the internal Vmidref located at node 129. Duringinitial calibration of servo position demodulator 22, the referencecurrent in AGC loop 34 is set so that the A/D converter 90 full digitalscale corresponds to one half of a servo track. The internal Vmidrefcorresponds to the track center or zero position error. The latched,most significant bit is applied to integrator 114. The output ofintegrator 114, external Vmidref at node 134, is driven towards thevalue of internal Vmidref. The gain through integrator 114 is chosen sothat external Vmidref just crosses the voltage level of internal Vmidrefbefore the next servo sector. This is shown in FIG. 12B. Thepeak-to-peak limits of external Vmidref must be kept much lower than theanalog value of a least significant bit of A/D converter 90. This keepsthe initial voltage on PES capacitor 84 as close as possible to theexact center of the digital scale.

After the first A/D conversion is completed, switch 118 is opened andA/D converter 90 is available for conversions 2, 3 and 4 which areconversions for the position samples read from the PES2A, PES1 and PES2Bfields. Between position samples, analog switch 118 closes to reset PEScapacitor 84 to external Vmidref. Hence, DC null loop 110 compensatesfor static or time variable DC off-sets in buffer 88 and analog switch118 which are inside DC null loop 110. This increases the trackfollowing accuracy of transducer 14.

The technique of time multiplexing A/D converter 90 between DC null loopoperation and PES conversions eliminates the need for two separate A/Dconverters to perform these tasks. This reduces the cost of servoposition demodulator 22.

As described earlier, after PES capacitor 84 is reset to externalVmidref, the output of voltage to current converter 72 is connected toPES capacitor 84 through analog switch 86 and the PES capacitor voltageramps up or down as shown in FIG. 8. The difference between the finalvoltage on PES capacitor 84 and the initial voltage is proportional tothe distance of the center of transducer 14 from the track center.

d. Correlation of Track ID With PES Information

FIG. 13A shows the concept of a total position word. The track ID numberand the digital value representing the PES field samples are combined todefine the radial position of transducer 14 anywhere on disk 16. Thetrack ID number defines the track and the PES samples define theposition within the track.

However, a problem can still arise. As transducer 14 flies over thetrack on disk 16, it moves over the track ID field prior to moving overthe PES fields. Hence, the track ID information is decoded earlier intime than information read from the PES fields. Since the informationread from the PES fields is read later in time, it is more recent and,hence, more accurate position information than that read from the trackID field. When transducer 14 is moving radially, it has a slantedtrajectory across the servo sector. Therefore, it may enter the servosector in the track ID field of one track but cross a boundary betweentracks and exit the servo sector from the PES field of the next track.This will result in an occasional mismatch between the track boundarydefined by the track identification information and that defined by thePES fields. It then becomes necessary to correct the track ID number sothat it corresponds to the track from which transducer 14 reads the PESfield samples.

In the present invention, correction logic 140 (shown in FIGS. 6 and13D) is used to detect when it is necessary to correct the track ID sothat it corresponds to the track number from which the PES field sampleswere read. FIGS. 13B1, 13B2 and 13B3 are graphs of the amplitude of theposition signal corresponding to the PES fields, in 2's complement form,as it relates to the least significant bit in the track ID signal. FIGS.13B1, 13B2 and 13B3 also show a corrected least significant bit in thetrack ID signal.

Trace A represents the 2's complement form of the amplitude of theposition signal corresponding to the PES1 field. Trace B represents the2's complement amplitude corresponding to the PES2 field (PES2A+PES2B).The truth table shown in FIG. 13C defines the conditions where the trackID information is not correlated with the PES field information and thecorrection word which is required.

Combinational logic 140, shown in FIG. 13D, forms a correction number of±1 which is added to the track ID in adder 142. For all other cases, thenumber 0 is added to the track ID.

FIG. 13E shows one preferred hardware implementation of combinationallogic 140. It should be noted that the addition of the correction wordto the track ID could also be accomplished by software in servo controlprocessor 24. This correlation allows servo control processor 24 to usebits of the digital composite PES signal along with a track ID to veryprecisely determine the position and velocity of transducer 14 even athigh radial velocities. The increased accuracy of the total positionsignal at high radial velocities helps to minimize both the time andpower required for performing a track seek operation. At low radialvelocities, such as at the end of a track seek operation, thiscorrelation permits a smoother settling of the transducer on track.

e. Forming Composite PES Signal

FIGS. 14A1, 14A2 and 14A3 are an imaginary tracing of digitized finalvoltage values of the analog PES ramps generated at PES capacitor 84 forall radial positions of transducer 14 across four adjacent tracks in aservo-on-data system. These imaginary tracings are called track crossingplots. The digital values are in 2's complement form and the point atwhich the track crossing plot for the PES1 field crosses 00 hexrepresents the track center.

Demodulation of the PES2A field produces the PES2A tracking crossingplot shown in graph G1 of FIG. 14A1. Similarly, demodulation of the PES1and PES2B fields produces the PES1 and PES2B track crossing plots,respectively. These three track crossing plots represent all possibleideal digital input combinations to the composite PES generator 92.

Graph G1 in FIG. 14A shows the three track crossing plots condensed ontoa single plane. Since there are equal number of cycles in field PES2Aand PES2B, their corresponding track crossing plots coincide. However,since the integration measurement time period for the PES2A and PES2Bfields is only one half that of the PES1 field, the slope of the PES2Aand PES2B track crossing plots in volts per microinch is only one halfthat of the PES1 track crossing plot.

In this invention, the PES2A position sample is added to the PES2Bposition sample to produce the total PES2 or quadrature sample. Onehardware implementation to perform this addition was discussed withreference to FIG. 6. This addition can also be performed graphically.Graph G2 in FIG. 14A2 shows that the addition of the PES2A and PES2Btrack crossing plots produces a new track crossing plot which has thesame slope as the PES1 track crossing plot.

The PES1 and PES2 track crossing plots shown in graph G2 in FIG. 14A2are the inputs to composite PES generator 92. However, at any one radialposition along disk 16, only one of the two plots is in a region wherethe PES amplitude is changing linearly with transducer position. Thereare eight distinct regions labeled a through h which repeat across thetrack. By monitoring the amplitude and polarity of the PES1 and PES2samples, composite PES generator 92 determines over which of the eightregions transducer 14 is flying. Composite PES generator 92 then createsa composite position error sample output which is the standardized formof position error sample required by servo control processor 24.Composite PES generator 92 accomplishes this by inverting the slope orchanging the sign bit of the PES1 or PES2 track crossing plots whennecessary, and combining the linear portions to create the desiredcomposite PES track crossing plot shown in graph G3 in FIG. 14A2.

FIGS. 15A through 15C show how the addition of the PES2A and PES2B fieldsamples helps to compensate for high radial velocity of transducer 14. Aslanted trajectory of transducer 14 is shown by arrow 150 in FIG. 15A.This slanted trajectory causes an apparent shift in the PES1 and PES2Btrack crossing plots as shown in FIG. 15B. The actual value of PES2A and2B is shown by a solid line and the ideal value is shown by a dashedline. By adding the PES2A track crossing plot with the shifted PES2Btrack crossing plot, a new track crossing plot, PES2C is formed as shownin FIG. 15C. The new track crossing plot PES2C has the correctorientation with respect to the PES1 track crossing plot. By using thistechnique, the composite PES output is generated correctly, independentof the radial velocity of transducer 14. Without this feature,nonlinearities in the composite PES output would occur due to theslanted trajectory of transducer 14. This allows more efficient actuatorcontrol during track seek operations.

This invention is not limited for use in dedicated servo systems such asthe one shown in FIG. 1. This invention may also be used in hybrid servoconfigurations. A hybrid system has a dedicated surface containing servosectors and other disks which contain one or more similar servo sectorsper revolution which are decoded for data transducer thermal off-setcompensation. Since a common servo sector pattern is used, through theuse of time multiplexing only one servo position demodulator is needed.

This invention may also be used in servo-on-data type servo systemswhere each disk surface has servo sectors. In this type of system,customer data is written between the write-protected servo sectors. Anyhead actively reading or writing customer data is also acting as its owntrack following error sensor. This increases track following precisionand allows higher track densities to be achieved.

Another servo system configuration in which this invention may be usedis one having multiple independent actuators 200. This is shown in FIG.17. If a multiple actuator system contains n actuators 200 andcorresponding amplifiers 202, the servo sectors under transducers 204 onadjacent actuators are staggered by 1/n of the distance between servosectors on one disk. The servo sectors for adjacent actuators must notoverlap. This is illustrated in FIG. 18. In this type of system, amultiplexer 204 continuously cycles through the servo sectors of theactuators. The advantage of this configuration is that only one controlprocessor 24, along with digital demultiplexer 206, D/A converters 208,amplifiers 202 and one servo demodulator 22 are needed to controlseveral actuators 200 (again through the use of time multiplexing).

The servo sector format described in this invention may be used withlinear or rotary actuators.

A method for creating the composite PES output for a dedicated servosystem is shown in FIGS. 16A1-16C. The servo track width can be twicethe data track width. The servo transducer 14 is then twice as wide as adata transducer 18. This can help minimize nonlinear effects of fringingfields read by the edges of transducer 14. It can also improve thesignal-to-noise ratio of the track ID bits. In this case, the compositePES generator 92 merely inverts the slope of the d, e, f and g segments.The servo track centers are derived alternately from PES1 and PES2samples.

For a servo-on-data system, the transducer is used for both serving andreading and writing customer data. The servo track and data track areautomatically the same width. The composite PES track crossing plot isthen formed as shown in graph G3 in FIG. 14A3. Slope segments a and dare repositioned to extend the linearity of the b and c segments. Thisproduces a linear PES plot across the full track width. Similarly, the eand h segments extend the linearity of the f and g segments. The trackcenter is always derived from the PES1 sample. One advantageous featureof this method is that the composite PES has one more bit ofquantization than the A/D converter used in the servo positiondemodulator 22.

CONCLUSION

In the present invention, the quadrature field is split in half. Half ofthe quadrature field wise on one side of the normal field and half lieson the opposite side of the normal field. By adding the information fromthe PES2A and PES2B fields, the total quadrature crossing plot iscorrectly oriented with respect to the normal track crossing plot. Thistechnique eliminates nonlinearities in the composite PES output due tothe slanted trajectory of transducer 14 as it moves radially withrespect to disk 16. In essence, by using this technique, the compositePES output is generated correctly independent of the radial velocity oftransducer 14. Thus, the actuator can be controlled more efficientduring track seek operations.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method of detecting transducer position errorof a transducer in a data storage system, comprising:providing aquadrature field and a normal field on a disk in the data storagesystem, the quadrature and normal fields each containing positioninformation, and the quadrature field being divided into a firstquadrature field and a second quadrature field, the first and secondquadrature fields being arranged relative to the normal field tocompensate for a temporal shift in position information read from thenormal field relative to position information read from the quadraturefield caused by radial velocity of the transducer over the disk; readingposition information from the first quadrature field on the disk in thedata storage system; reading position information from the normal fieldon the disk to produce a normal position word, the first quadraturefield being located on a first side of the normal field; readingposition information from the second quadrature field, the secondquadrature field being located on a second side of the normal field, thesecond side being opposite the first side; and adding the positioninformation from the first quadrature field with the positioninformation from the second quadrature field to produce a totalquadrature word, the total quadrature word and the normal position wordrepresenting the transducer position error.
 2. The method of claim 1 andfurther comprising:combining the normal position word with the totalquadrature word to produce a composite position error samplerepresenting transducer position error.
 3. The method of claim 2 andfurther comprising:reading position information from a trackidentification field on the disk to produce a track ID word.
 4. Themethod of claim 3 and further comprising:combining the track ID wordwith the composite position error sample to produce a total positionword representing total transducer position.
 5. The method of claim 1and further comprising:providing the normal field having a length on thedisk in the data storage system; providing the first quadrature fieldhaving a length on the disk which is one half the length of the normalfield; and providing the second quadrature field, having a length on thedisk which is one half the length of the normal field.
 6. A sector on adisk containing position information used to determine position error ofa transducer over the disk in a data storage system comprising:a normalfield having a length and having a first side and a second side, thefirst side being oriented opposite the second side on the disk; a firstquadrature field, having a length which is half the length of the normalfield, located on the first side of the normal field; and a secondquadrature field, having a length which is half the length of the normalfield, located on the second side of the normal field, the normal fieldand the first and second quadrature fields containing positioninformation and being arranged relative to the normal field tocompensate for temporal shifts in position information read from thenormal field relative to position information read from the quadraturefields caused by radial velocity of a transducer over the disk.
 7. Anapparatus for detecting transducer position error of a transducer in adata storage system, comprising:a storage disk having a sectorcontaining position information, the sector including a normal fieldhaving a length and having a first side and second side, the first sidebeing oriented opposite the second side on the disk, a first quadraturefield having a length which is half the length of the normal field andlocated on the first side of the normal field, and a second quadraturefield having a length which is half the length of the normal field andlocated on the second side of the normal field; first reading means forreading position information from the first quadrature field on the diskin the data storage system; second reading means for reading positioninformation from the normal field on the disk to produce a normalposition word; third reading means for reading position information fromthe second quadrature field; and first combining means for combining theposition information from the first quadrature field with the positioninformation from the second quadrature field to produce a totalquadrature word, the first combining means combining the positioninformation to compensate for shifts between the position informationfrom the normal and quadrature fields caused by radial velocity of thetransducer over the disk, the total quadrature word and the normalposition word representing the transducer position error.
 8. Theapparatus of claim 7 and further comprising:second combining means forcombining the normal position word with the total quadrature word toproduce a composite position error sample representing transducerposition error.
 9. The apparatus of claim 8 and furthercomprising:fourth reading means for reading position information from atrack identification field on the disk to produce a track ID word. 10.The apparatus of claim 9 and further comprising:third combining meansfor combining the track ID word with the composite position error sampleto produce a total position word representing total transducer position.11. A method of detecting transducer position error of a transducer in adata storage system, comprising:providing a normal field having a lengthon a disk in the data storage system; providing a first quadrature fieldhaving a length on the disk which is one half the length of the normalfield; providing a second quadrature field, having a length on the diskwhich is one half the length of the normal field; reading positioninformation from a first quadrature field on the disk in the datastorage system; reading position information from the normal field onthe disk to produce a normal position word, the first quadrature fieldbeing located on a first side of the normal field; reading positioninformation from the second quadrature field, the second quadraturefield being located on a second side of the normal field, the secondside being opposite the first side; and adding the position informationfrom the first quadrature field with the position information from thesecond quadrature field to produce a total quadrature word, the totalquadrature word and the normal position word representing the transducerposition error.
 12. The method of claim 11 and furthercomprising:combining the normal position word with the total quadratureword to produce a composite position error sample representingtransducer position error.
 13. The method of claim 12 and furthercomprising:reading position information from a track identificationfield on the disk to produce a track ID word.
 14. The method of claim 13and further comprising:combining the track ID word with the compositeposition error sample to produce a total position word representingtotal transducer position.